The present invention relates generally to magnetoresistive random access memory (MRAM), and more particularly to MRAM cells having magnetic tunnel junction (MTJ) units with continuous tunnel layers.
MRAM is a type of memory device containing an array of MRAM cells that store data using their resistance values instead of electronic charges. Each MRAM cell includes a magnetic tunnel junction (MTJ) unit whose resistance can be adjusted to represent a logic state “0” or “1.” Conventionally, the MTJ unit is comprised of a fixed magnetic layer, a free magnetic layer, and a tunnel layer disposed there between. The resistance of the MTJ unit can be adjusted by changing the direction of the magnetic moment of the free magnetic layer with respect to that of the fixed magnetic layer. When the magnetic moment of the free magnetic layer is parallel to that of the fixed magnetic layer, the resistance of the MTJ unit is low, whereas when the magnetic moment of the free magnetic layer is anti-parallel to that of the fixed magnetic layer, the resistance of the MTJ unit is high. The MTJ unit is coupled between top and bottom electrodes, and an electric current flowing through it from one electrode to another can be detected to determine its resistance, and therefore its logic state.
FIG. 1 illustrates a cross-sectional view of a typical MRAM cell 100 comprised of a MTJ unit 102 coupled to a bit line 104 through a top electrode 106, and to a source/drain doped region 108 of a MOS device 116 through a bottom electrode 110 and a contact 112. A write line 114 is placed underneath the MTJ unit 102 for generating an electromagnetic field to change the resistance of the MTJ unit 102 during write operation. During read operation, the MOS device 116 is selected to pass a current through the bit line 104, the top electrode 106, the MTJ unit 102, the bottom electrode 110, and the contact 112 to its source region 118. The current detected at the bit line 104 is compared with a reference to determine whether the resistance of the MTJ unit 102 represents a high or low state. Because MRAM does not utilize electric charges for data storage, it consumes less power and suffers less from current leakage than other types of memory, such as static random access memory (SRAM), dynamic random access memory (DRAM) and flash memory.
FIGS. 2-4 illustrate cross-sectional views of a MTJ unit in progress during a fabrication process. Referring to FIG. 2, a stack of bottom conductive layer 202, anti-ferromagnetic layer 204, pinned layer 206, tunnel layer 208, free magnetic layer 210 and top conductive layer 212 is formed above a semiconductor substrate (not shown in the figure). The anti-ferromagnetic layer 204 fixes of the magnetic moment of the pinned layer 206 in one direction, whereas the magnetic moment of the free magnetic layer 210 can be changed by applying external electromagnetic forces. A photoresistor layer 214 is formed on the top conductive layer 212 to define a width of the MTJ unit in progress.
An etching processing using the photoresistor layer 214 as a mask is performed to remove parts of the top conductive layer 212 uncovered by the photoresistor layer 214. The photoresistor layer 214 is then stripped after the etching process reaches the top surface of the free magnetic layer 210, rendering a cross-sectional view as shown in FIG. 3.
Another etching process, preferably dry etching, is performed using the top conductive layer 212 as a hard mask to remove the free magnetic layer 210, the tunnel layer 208, the pinned layer 206 and the anti-ferromagnetic layer 204 uncovered by the top conductive layer 212 in order to separate a MTJ unit from its neighboring units. The etching process stops when it reaches the top surface of the bottom conductive layer 202, rendering a cross-sectional view as shown in FIG. 4.
One drawback of the conventional etching process in forming the MTJ unit is that the MTJ unit is susceptible to a reliability issue of short circuit. The etching process is performed in a chamber where plasma is introduced to bombard the surface of the MTJ unit in progress. As a result, there may be residual conductive materials remaining on sidewalls of the completed MTJ unit as shown in FIG. 4. These residual conductive materials may conduct a current between the bottom conductive layer 202 and the top conductive layer 212 bypassing the tunnel layer 208, thereby causing the MTJ unit to fail.
Another drawback of the conventional etching process in forming the MTJ unit is that the top conductive layer 212 and the photoresistor layer 214 need to be thick. The MTJ unit is relatively deep for purposes of etching as it is comprised of layers including the free magnetic layer 210, the tunnel layer 208, the pinned layer 206, and the anti-ferromagnetic layer 204. Because the top conductive layer 212 as a hard mask is consumed during the etching process, it needs to be sufficiently thick to ensure that enough of it will remain on the free magnetic layer 210 after the etching. Likewise, the photoresistor layer 214 needs to be sufficiently thick to ensure that enough of it will remain on the top conductive layer 212 after its etching. This poses a challenge to MRAM fabrication, especially when MRAM continues to shrink in size beyond 45 nm of conductor width.
Yet another drawback of the conventional etching process in forming the MTJ unit is that the top surface of the top conductive layer 212 may become rounded after the etching, thereby increasing the difficulty of forming a contact thereon. During the etching process, the corners of the top conductive layer 212 are etched off faster than other parts. As a result, it may be difficult to properly form a contact on the conductive layer 212, and thus causing reliability issues.
As such, what is needed is a method of fabricating MRAM that addresses the short circuit and mask thickness issues present in the conventional process.